This invention relates to seismic exploration, and more particularly, to a method of enhancing seismic sections obtained in areas where shear waves are detected.
In seismic exploration, seismic energy is generated by a source located at or near the surface of the earth, is reflected from subsurface interfaces between layers of the earth, and is received by a spread of seismic detectors having different offsets from the source on the surface of the earth. The detected seismograms are gathered, stacked, and recorded in the form of a set of common deth point (CDP) seismic traces as illustrated, for example, in FIG. 1 of U.S. Pat. No. 4,206,509--Ruehle. From these recorded CDP seismic traces the subsurface interval velocities can be determined. Such a CDP gathering and recording technique and a continuous velocity determination technique are disclosed in U.S. Pat. No. 3,651,451--Ruehle. An accurate velocity determination is quite important in the determination of the nature and extent of the subsurface layering.
While CDP exploration techniques have been very successfully used, many unexplained phenomena occur on seismic sections. For example, many seismic sections have low frequency interfering sets of reflections which have the appearance and velocity of multiple reflections. Other phenomena which have evoked a great deal of interest in the exploration for hydrocarbons are the so called "bright spots" or HCI (hydrocarbon indicators) which appear on some seismic sections.
In accordance with an important aspect of my invention, I have demonstrated that many previously unexplained phenomena on seismic sections are caused by the conversion of longitudinal waves (p waves) to shear waves (s waves) at subsurface interfaces.
The existence of longitudinal waves and shear waves in the propagation of seismic energy through the earth is well known. This phenomenon has been well identified and used in well logging, where an acoustic transmitter and detector traverse a bore hole through the earth. See, for example, U.S. Pat. No. 3,333,238, Caldwell.
In seismic exploration shear waves are produced by some sources, notably vibratory seismic sources. However, studies on shear wave reflections in seismograms have not been fruitful. Except where the p wave to s wave conversion occurs near the surface and a symmetrical travel path exists, conventional CDP methods for enhancing the reflections cannot be applied. Furthermore, the s wave attenuation for deep arrivals reduces the amplitudes far below those of multiple and primary longitudinal reflections.